It is well known to those skilled in the magnetic recording systems art that two principal design objectives in any mass or bulk recording system are high data storage density and high data retrieval accuracy. Recording density is substantially a function of the physical proximity with which magnetic flux transitions can be placed in the bulk nagnetic medium whereas accuracy of readout involves, among other things, the establishment of an accurate timing signal so as to determine those points at which the read circuitry should be sensitized so as to observe a flux condition or a transition between flux conditions.
A number of data recording codes and systems have been developed to satisfy one or both of these basic design objectives in varying degrees. The highest density recording cam be achieved using a so-called non-return to zero (NRZ) code wherein all binary ZEROS are represented by one flux condition and all binary ONES are represented by another flux condition. While this code is efficient, it has the inherent disadvantage of requiring auxiliary clocking signals since a long string of data cells of one or the other value presents no flux transitions to develop clock pulses. Other recording codes have been worked out which are of lesser efficiency; that is, codes which require more than one flux transition per recorded data bit, but which are self-clocking in nature and, thus, eliminate some complexity in the timing system.